LNB Position & Angle Setup Problem

garym71

Before mounting my fixed-dish with multi-LNB on the wall, I decided it would probably be a good idea to set the LNB mountings to their correct positions first. The problem is that unless my calculations are wrong, I seem to have an issue with not being able to get one of the LNB holders close enough for the required angle alignment...

(click for full size picture)
LNBHolderPositions.jpg

The length of the LNB arm from the dish to the centre of the LNB holder in a straight line (green line) is 470mm. The LNB on the end of this will be pointing at 19E.

The LNB on the left-hand side (blue line) will be for 28.2E so the angle between the green and blue lines at "A" needs to be 9.2°. This means that the distance of the LNB holder from the central holder (dimension "B") needs to be 76.1mm. This is no problem and I have set that LNB holder in the correct position successfully.

The LNB on the right-hand side (red line) will be for 13E so the angle between the green and red lines at "A" needs to be 6°. This means that the distance of the LNB holder from the central holder (dimension "C") needs to be 49.4mm. This is where the problem lies as the closest I physically can get that LNB holder to the central one on the arm is 60mm. This is obviously going to mean the angle of that LNB to the centre of the dish will be incorrect (7.28°).

To calculate the correct positions of the LNB holders I used an online triangle calculator. Am I doing something wrong here or is there some way around my problem that I'm not aware of?

The dish is a Raven Gemini 70cm dish with the standard Raven east/west LNB holders attached. I'm sure there must be other people running a 13E / 19E / 28.2E setup with the same dish so I would be interested to know how to do it.

Any help / advice on what I can do to resolve this problem would be much appreciated.

Gary

rolfw

I normally only use the Eastern sats offset, using hotbird in the centre, you may not be spot on with central focus on the 19E LNB, but it will work.

I tend to adjust the offset LNBs using a meter.

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garym71

I see what you're saying but I'm still not entirely sure how that would work

If I point the centre LNB (directly on the end of the arm) to Hotbird 13E then on the eastern offset bracket (the left-hand one), the next LNB along for Astra 19E will still need to be 49.4mm from the centre arm LNB holder (to the immediate left) which is simply not possible due to the width of the holders.

Am I missing something here with how the holder position locations should actually be calculated & set or is the real solution to start modifying the LNB holders? (i.e. try to slim them down by filing siome of the side off)

member91040

Bros is there any kind of software which can help our bro garym, To hold multi lnb. calculation.

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rolfw

You don't need software, I have set up many of these dishes with Eastern sats offset bracket, it works perfectly.

The bracket needs to be bent inward slightly and upwards, the Astra 19.2 LNB will be just touching the Hotbird LNB, see example below.

Attached Images

cmravengeminidishwitheasternsatellitesoffsetarmanddiseqcswitch.jpg (60.2 KB, 24 views)

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johnsattuk

You are using the position of the satelite on the equator as your pointing angle. The actual pointing angle will be different (generally greater) than this. One of the online caculators will give you the actual pointing angle for each sat. from your location, which you can then use in your calcs.

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Oldgeezer

garym71

OK, having taken in the replies and after doing some further research, I think I now understand things a bit better

Using dishpointer I've found out the that my location requires the following azimuth's...

• 28.2E Astra 2 (Sky UK) = 144.3°
• 19.2E Astra = 154.6°
• 13.0E Hot Bird = 162.1°

So it seems that I have an extra 1.5° to play with between the 13E and 19.2E LNB holders than I previously thought (7.5° between them instead of the expected 6°). This equuates to a distance of 61.9mm between the 13E and 19.2E LNB holders (instead of my 49.4mm) so with those calculations I can just about physically position the two holders that distance apart without any modifications.

As Rolf has already stated, I might not be able to get dead on the centre of the dish from the 19.2E LNB due to the close proximity of the 2 LNB feedhorns but I can set the calculated LNB holder positions before I mount the dish and then play around slightly with minor adjustments to get the best signal once the dish is actually on the wall. I'm sure I'll think of some more questions again before that though

Thanks guys